The terms “stringer” of stairs and the term “flight” of stairs are used interchangeably herein and both refer to a series of plural interconnected treads and risers (steps) that are placed to extend between vertically spaced apart levels of a structure to provide access and egress thereto. It is expressly noted herein that such “stringers” or “flights” of stairs need not be linear from end-to-end and may have curves, bends and shapes.
In the building of multi-floor structures, such as buildings, parking garages and other applications that require concrete steps it is common that the vertical distance between adjacent floors or levels is not consistent from floor to floor and in many cases the variance between floors may vary from fractions of an inch to several feet. The variance in vertical heights between the floors makes it difficult, and at times impossible, to use preformed standardized stringers of stairs to extend between the floors because it is imperative that all walking surfaces (including stair treads and landings) be parallel and that each tread is separated by a riser of identical height. Building codes typically limit height variance to ¼″ between adjacent treads and ⅜″ across an entire flight of consecutive steps. In instances when the variance between adjacent levels of the structure is small, such as less than ⅜ inch, a standardized stringer of stairs may be used, but will necessitate that the standardized stringer of stairs be both tipped a first direction to “stretch” to a greater vertical/horizontal distance or tipped a second direction to “shrink” to a lesser vertical/horizontal distance, and the landings at the upper and lower ends of the stringer of stairs thereafter need to be modified (ground) or (added to) to prevent gaps or raised edges that are tripping hazards. Although tipping a stringer of stairs is a common and an accepted practice to accommodate small vertical height variances in the vertically spaced levels, the tipping of the stringer of stairs has an additional negative effect of causing the stair treads and the upper and lower landings to not be horizontal which may cause the stringer of stairs to be noncompliant with building codes and increase risk of premises liability if a user were to trip or slip on the stairs. This may be an extreme risk in situations where the tipped stringers of stairs are used in unheated areas such as parking garages where moisture (rain/snow) may freeze to become ice.
In the construction a multi-floor structures, concrete stairways are a preferred means for providing non-mechanized access to the vertically spaced apart floors. Concrete is the preferred medium for building such stairways because it is strong, robust, durable, requires little maintenance, does not require use of separate fasteners, and is substantially “quieter” when being used by pedestrians ascending or descending the stairs. Further, concrete stairways may be integrated into the vertically spaced concrete decks of the structure. Unfortunately, concrete stairways are expensive because they are difficult to form and support during the construction process. Much of this cost is related to the labor necessary to create the forms and to provide support/shoring of the forms before and while the concrete is added to the forms. This is especially true if the stairway is external to the structure being built (such as a stairway immediately adjacent an exterior surface of the building).
Traditionally, when concrete, or similar formable or settable material is used to construct stairs, the stairs are either “cast-in-place”, which is an expensive and time consuming process, or a stringer of stairs is “pre-cast” and the entire unit is set in place (installed) after fabrication.
Because concrete stairways are so difficult to construct and expensive to construct, designers, engineers and architects have compromised to provide some of the benefits of concrete stairways, while eliminating the excessive costs and difficulty of building/installing such stairways. The compromise has been steel stringers that may or may not have concrete treads. The steel/concrete combination stairways are less expensive, but come with drawbacks including excessive maintenance (such as painting) and corrosion treatment, steel stringers require separate fastening means which are typically mechanical (bolts/screws and anchors) and steel stairways are notoriously noisy when being used by pedestrian traffic.
The instant invention is an improvement in the process of forming a “pre-cast” concrete stringer of stairs. To form pre-cast stringers of stairs, a mold is made which is a negative of the “stringer” of stairs to be formed. Concrete is poured into the mold. The concrete is allowed to harden and the resulting stringer of stairs is harvested from the mold. Producing a dimensionally precise negative is very difficult and expensive, especially for single use applications.
In the world of pre-cast concrete products, everybody is striving for perfection, but such perfection is very difficult and expensive to obtain. Limitations in form material, manufacturing processes and human error cause variances upon variances that often cause the pre-cast component to be out of tolerance. On-site accommodations are not a favorable “fix” which is why pre-cast strings of stairs are not always the selection of first choice for engineers and architects.
The instant invention is directed to a method and apparatus for precision forming concrete into desired shapes. More specifically, the invention relates to a new and improved concrete forming device which can be assembled from precision pre-cut component parts, used to impart a precise desired shape to a flowable concrete mixture, removed when the concrete has hardened, disassembled and moved to a new location for use, or reused to form another stringer of stairs.
As is well known, freshly mixed concrete is flowable and must be retained in some type of forming device until it has hardened or “set” if it is to achieve the structural shape desired by an end user. A number of methods in the past have been employed to do this. Among available forms are wood forms, fiber forms, steel forms and fiberglass forms. Forms constructed from wood are reasonably inexpensive and relatively easy to work with. However, wood forms are porous and frequently have rough surfaces. These factors create a tendency for concrete to adhere to the forms, not only making it difficult to remove the forms after the concrete has set, but also making it hard to reuse the forms because portions of the surface often become partially coated with hardened cement. Further any surface texture of the forms, such as wood grain, cracks and the like are transferred to the hardened concrete which may cause negative aesthetic impressions. The need to frequently replace the forms and the effort required to disassemble and remove them, creates an appreciable expense over time. Steel forms generally comprise segments that are fabricated into predetermined units. Various problems with steel sectional forms include heavy weight, expensive production, difficult modification, the possibility of rusting steel, as well as the same tendency of concrete adherence that wooden forms have. Since steel forms are expensive they cannot be discarded, but must be thoroughly cleaned for reuse. This is a time consuming and costly process.
With Cast-in-Place concrete stairway systems, it has been common practice to construct formwork at a building site to receive the flowable concrete so as to form the stair structure. The formwork is typically made of wood and discarded once the concrete has been poured and has hardened. Wood formworks need to be braced, shored, and otherwise reinforced, thus, the building of formwork for concrete stairs requires much time, effort and materials which need to be repeated for each flight of stairs. Cast-in-Place concrete stairs are an expensive alternative, and because of that are often not used if other less expensive alternatives are available. The method and apparatus of the present invention can advantageously be used to build a formwork for a stringer of stairs of various sizes and shapes, with various structural and aesthetic elements. The typical stringer of stairs consists of a number of stairs and usually a landing or platform at the bottom and at the top. Each individual step consists of a tread and a riser. The “tread” is the part of the stair that is oriented generally horizontally and is stepped on by a user's foot. The “riser” is the part of the stair that is oriented generally vertically and connects each tread to an adjacent tread or a tread to an adjacent landing. Each tread may optionally carry a nosing which is positioned at a toe edge of the tread opposite the riser, along its width and is part of the tread that protrudes outwardly over the riser beneath.
Pursuant to industry building codes, each stair is required to have a “back set” from toe-to-toe which requires the riser to extend outwardly from the vertically adjacent below riser, or which requires the riser to be angulated relative to the interconnecting treads to provide the required “back set”. Such angulation can significantly increase fabrication time because the angles are complex and must be precise to retain the flowable concrete within the form.
The overall height of a flight of stairs is called the Overall Rise (OR) and the overall length of the stairs is the Run-Length (RL). The rise height (RH) of each stair is measured from the top of one tread to the top of the next adjacent tread. The ratio of Rise Height (RH) to Run-Length (RL) is the pitch (P). The width (W) of each tread is measured from a first side edge to a second side edge of the same tread, and the tread depth (TD) is measured from the outer (front toe) edge of the nosing to the riser on the opposite (back) edge of the same tread, which is known as the “heel” of the stair. The Going (C) of a stair is the horizontal distance from the edge of the nosing of a tread to the edge of the nosing of the adjacent tread. The number of stairs in a flight of stairs is deduced by the number of risers present. The “throat” of the stair is the perpendicular distance from the back of the stringer to the “heel” of a stair. The “Throat” of the stair varies in thickness depending on the structural and/or aesthetic requirements of the stair.
One of the drawbacks to concrete flights of stairs is that they are difficult to properly produce, particularly if the stairway is wide, has additional aesthetic elements, is built “in-space” (as opposed to mid-slab or inside a core wall), or has a large number of risers. The concrete is initially in a flowable state and must be held in place by a form. If the stairway is large, the flowable concrete will present a substantial load on the form. Concrete will need to be vibrated during the pouring process to ensure the concrete is properly consolidated. The vibration presents an additional loading on the forms. As the concrete cures, the exposed surfaces of the concrete must be finished to provide the desired surface texture.
What is needed is an apparatus that can be used to form concrete flights of stairs that is reusable, one that can handle the loads associated with large stairs, one that facilitates the pouring and finishing of the stairs, and one that is easily configurable to handle a variety of different stair configurations.
The present invention relates to construction forms and more specifically to a method and apparatus for making precision forms for making precision concrete flights of stairs.